Curiously enough, I just checked a vanilla installation and there is no OK button on the CHECKIN_CONFIRM_FORM. The only button I see is 'Check In Similar'. Any chance someone has already customized that?

You can check it easily: see http://docs.oracle.com/cd/E14571_01/doc.1111/e11011/c05_core003.htm#BABHAIAH , find the relevant template (checkin_confirm.htm) and you will see what's in there / alternatively you can create a component which customizes that accordingly.